U.S. patent application number 11/683859 was filed with the patent office on 2007-09-20 for probe and near-field microscope.
This patent application is currently assigned to CANON KABUSHIKI KAISHA. Invention is credited to Ryota Sekiguchi.
Application Number | 20070216422 11/683859 |
Document ID | / |
Family ID | 38517145 |
Filed Date | 2007-09-20 |
United States Patent
Application |
20070216422 |
Kind Code |
A1 |
Sekiguchi; Ryota |
September 20, 2007 |
PROBE AND NEAR-FIELD MICROSCOPE
Abstract
A probe includes a tubular conductor having an aperture at one
end thereof. An electromagnetic wave transmitting unit for
transmitting an electromagnetic wave, via the tubular conductor, to
a position distant from the aperture is disposed at one of the
inside and the outside of the tubular conductor, and an
electromagnetic wave receiving unit for receiving an
electromagnetic wave, via the tubular conductor, from the position
distant from the aperture is disposed in the other of the inside
and the outside of the tubular conductor. The size of the aperture
is smaller than or equal to the wavelength of the electromagnetic
waves. The electromagnetic waves transmitted and received at the
outside and the inside of the tubular conductor are coupled through
the aperture. When an analyte to be observed is disposed so as to
face the aperture, information of the analyte is obtained on the
basis of a change in the coupling of the electromagnetic waves
through the aperture.
Inventors: |
Sekiguchi; Ryota;
(Kawasaki-shi, JP) |
Correspondence
Address: |
FITZPATRICK CELLA HARPER & SCINTO
30 ROCKEFELLER PLAZA
NEW YORK
NY
10112
US
|
Assignee: |
CANON KABUSHIKI KAISHA
Tokyo
JP
|
Family ID: |
38517145 |
Appl. No.: |
11/683859 |
Filed: |
March 8, 2007 |
Current U.S.
Class: |
324/637 |
Current CPC
Class: |
G01N 21/3581 20130101;
G01Q 60/22 20130101; G01N 21/9501 20130101 |
Class at
Publication: |
324/637 |
International
Class: |
G01R 27/32 20060101
G01R027/32; G01R 27/04 20060101 G01R027/04 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 17, 2006 |
JP |
2006-073614 |
Claims
1. A probe comprising: a tubular conductor having an aperture at
one end thereof; an electromagnetic wave transmitting unit for
transmitting an electromagnetic wave, via said tubular conductor,
to a position distant from the aperture; and an electromagnetic
wave receiving unit for receiving an electromagnetic wave, via said
tubular conductor, from the position distant from the aperture,
wherein the electromagnetic wave transmitting unit is disposed at
one of the inside and the outside of the tubular conductor, the
electromagnetic wave receiving unit is disposed at the other of the
inside and the outside of the tubular conductor, the size of the
aperture is smaller than or equal to the wavelength of the
electromagnetic waves, the electromagnetic waves transmitted and
received at the outside and the inside of the tubular conductor are
coupled through the aperture, and when an analyte to be observed is
disposed so as to face the aperture, information of the analyte is
obtained on the basis of a change in the coupling of the
electromagnetic waves through the aperture.
2. The probe according to claim 1, further comprising a dielectric
material inside the tubular conductor.
3. The probe according to claim 1, further comprising an
electromagnetic wave coupling unit outside the tubular conductor,
for regulating the efficiency of coupling the electromagnetic waves
with the free space.
4. The probe according to claim 1, wherein the end of the tubular
conductor having the aperture is tapered.
5. The probe according to claim 1, wherein the electromagnetic
waves include part of the frequency region from 30 GHz to 30
THz.
6. A near-field microscope comprising the probe according to claim
1 and a position control system for controlling the relative
positional relationship between the probe and the analyte.
7. An analyte observing method for obtaining information of an
analyte using the probe according to claim 1, the method comprising
the step of making an electromagnetic wave that is a traveling wave
exist in the inner part of the tubular conductor.
8. An analyte observing method for obtaining information of an
analyte using the probe according to claim 1, the method comprising
the step of making an electromagnetic wave that is a standing wave
exist in the inner part of the tubular conductor.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a probe and a near-field
microscope capable of obtaining information, such as dielectric
properties, of an analyte using electromagnetic waves. More
specifically, the present invention relates to a probe and a
near-field microscope capable of observing physical properties,
such as dielectric properties, of a minute region of an analyte
using electromagnetic waves in the frequency region from the
millimeter waveband to the terahertz band (30 GHz to 30 THz)
(hereinafter also referred to as "high-frequency electrical
signal").
[0003] 2. Description of the Related Art
[0004] As a method for observing dielectric properties of a minute
region of an analyte or a minute analyte, there is known a method
in which the reflection of a high-frequency electrical signal from
an analyte is measured using a high-frequency transmission line,
such as a coaxial line or a high-frequency co-planar strip line,
with a minute tip. This method is used in a wide frequency region
from the microwave region to the visible region, and is called
microwave probe (or microwave probe microscope) in the microwave
region. Since a microwave probe uses a minute structure smaller
than the wavelength, dielectric properties of a region smaller than
the wavelength of the high-frequency electrical signal to be used
can be measured. Therefore, the distribution of dielectric
properties of an analyte can be imaged with high spatial
resolution. In addition, the information of dielectric properties
of an analyte can be read from the phase retardation and the
decrease in amplitude of the high-frequency electrical signal
reflected from the analyte. The phase retardation reflects the real
part of the dielectric constant of the analyte, and the decrease in
amplitude reflects the imaginary part of the dielectric constant of
the analyte. Therefore, by analyzing these, dielectric properties
of the analyte can be quantitatively evaluated.
[0005] Japanese Patent Laid-Open No. 2005-121422 discloses an
apparatus that measures the complex dielectric constant of an
analyte with a microwave probe according to the above-described
principle. In the configuration of Japanese Patent Laid-Open No.
2005-121422, a generator transmits a high-frequency electrical
signal to a coaxial line with a minute tip, and a reflected wave
from an analyte is received by a detector via a directional
coupler. In addition, it is also possible to provide a resonance
structure of the high-frequency electrical signal in a
high-frequency transmission line so as to make a high-frequency
signal reflected from the analyte a standing wave, and to use a
shift of resonance frequency or a change in resonator Q-value with
the change in dielectric properties of the analyte, for
imaging.
[0006] Japanese Patent Laid-Open No. 2002-189043 also discloses an
apparatus that measures the complex dielectric constant of an
analyte with a microwave probe according to the above-described
principle. In the configuration of Japanese Patent Laid-Open No.
2002-189043, a generator transmits a high-frequency electrical
signal to a multi-conductor transmission line with a minute tip via
a first coupling probe, and a reflected wave from an analyte is
received by a detector via a second coupling probe. By
appropriately terminating the tip and the opposite end of the
multi-conductor transmission line, the reflected wave from the
analyte is made a standing wave in the multi-conductor transmission
line.
[0007] The spatial resolution of these microwave probes is 1/1000
or less of the wavelength of the microwave. Therefore, they are
so-called near-field probes (or near-field probe microscopes) in
the visible region.
[0008] There are a variety of high-frequency transmission lines.
Kanglin Wang, Daniel M. Mittleman: Nature, vol. 432 (2004)
discloses a wire waveguide consisting of a single conductor. The
wire waveguide features a capability of transmission of a
high-frequency electrical signal across a comparatively wide
frequency band. For example, it is known that its propagation loss
is smaller than those of other multi-conductor high-frequency
transmission lines in the frequency region from the millimeter
waveband to the terahertz band, and its dispersion is also
comparatively small.
[0009] However, in the microwave probe of the Japanese Patent
Laid-Open No. 2005-121422, it is necessary to use a directional
coupler or the like in order to branch the reflected wave of the
high-frequency electrical signal from the analyte. This complicates
the apparatus. In addition, in the frequency region from the
millimeter waveband to the terahertz band, when the frequency of
the high-frequency electrical signal to be used is high, the
propagation loss in the high-frequency transmission line is not
negligible. In the multi-conductor high-frequency transmission line
as in Japanese Patent Laid-Open No. 2002-189043, the propagation
loss is comparatively great, and therefore the sensitivity is low.
Kanglin Wang, Daniel M. Mittleman: Nature, vol. 432 (2004)
discloses only a wire waveguide consisting of a single
conductor.
SUMMARY OF THE INVENTION
[0010] The present invention provides a probe and a near-field
microscope with simple composition and high sensitivity.
[0011] In an aspect of the present invention, a probe includes a
tubular conductor having an aperture at one end thereof; an
electromagnetic wave transmitting unit for transmitting an
electromagnetic wave, via the tubular conductor, to a position
distant from the aperture; and, an electromagnetic wave receiving
unit for receiving an electromagnetic wave, via the tubular
conductor, from the position distant from the aperture. The
electromagnetic wave transmitting unit is disposed at one of the
inside and the outside of the tubular conductor, and the
electromagnetic wave receiving unit is disposed at the other of the
inside and the outside of the tubular conductor. The size of the
aperture is smaller than or equal to the wavelength of the
electromagnetic waves. The electromagnetic waves transmitted and
received in the outside and the inside of the tubular conductor are
coupled through the aperture. When an analyte to be observed is
disposed so as to face the aperture, information of the analyte is
obtained on the basis of a change in the coupling of the
electromagnetic waves through the aperture. Typically, the
electromagnetic waves used in this probe include part of the
frequency region from 30 GHz to 30 THz.
[0012] In another aspect of the present invention, a near-field
microscope includes the above-described probe and a position
control system for controlling the relative positional relationship
between the probe and the analyte.
[0013] In another aspect of the present invention, an analyte
observing method for obtaining information of an analyte using the
above-described probe or near-field microscope includes the step of
making an electromagnetic wave that is a traveling wave or a
standing wave exist in the inner part of the tubular conductor.
[0014] In the probe and the near-field microscope in the present
invention, a tubular conductor serves as an electromagnetic wave
transmission line. The inner part of the conductor is used as a
waveguide, and the outer part of the conductor is used as a wire
waveguide. Therefore, the tubular conductor in the present
invention functions as two electromagnetic wave transmission lines,
and the electromagnetic wave transmitted to an analyte can be
separated from the electromagnetic wave reflected from the analyte.
In order to separate these electromagnetic wave transmission lines,
the conductor has a wall thickness sufficiently larger than the
penetration depth into the conductor due to the skin effect of the
electromagnetic wave. For example, in the case of a high-frequency
electrical signal in the terahertz band, the tubular conductor has
a wall thickness larger than or equal to a micron. At one end of
such a tubular conductor, an aperture is formed. An analyte is
disposed at the end of the tubular conductor, and the reflection of
an electromagnetic wave from the analyte is measured. The analyte
can be measured, for example, by transmitting an electromagnetic
wave from a wire waveguide in the outer part of the conductor to
the end of the conductor where the analyte is disposed, and guiding
part of the electromagnetic wave reflected from the analyte to a
waveguide in the inner part of the conductor.
[0015] Since the inner structure of the tubular conductor in the
present invention is a waveguide, electromagnetic waves on the
higher frequency side than the cutoff frequency can be transmitted.
For example, in the case of a high-frequency electrical signal in
the terahertz band, the inner diameter of the tubular conductor is
at least 1 mm. Since the propagation loss in waveguides is
generally smaller than those of other high-frequency transmission
lines, the probe and the near-field microscope in the present
invention have advantages. In addition, the operating frequency
band of the probe and the near-field microscope according to the
present invention is typically on the higher frequency side than
the millimeter waveband. Therefore, the constraint due to the
cutoff frequency of the waveguide does not matter for size
reduction. For example, in the case where a high-frequency
electrical signal in the millimeter waveband is used, the inner
diameter of the waveguide is at most a few millimeters.
[0016] Since the outer part of the tubular conductor in the present
invention is a wire waveguide, a high-frequency electrical signal
can be transmitted across a wide frequency band. It is known that,
as shown in Kanglin Wang, Daniel M. Mittleman: Nature, vol. 432
(2004), the conductor loss in a wire waveguide is small
particularly in the frequency region from the millimeter waveband
to the terahertz band, and a wire waveguide is superior to other
multi-conductor high-frequency transmission lines. Therefore, this
point is also an advantage of the probe and the near-field
microscope in the present invention. Even in a wire waveguide whose
length in the transmission direction is more than a dozen
centimeters, the loss and dispersion of the high-frequency
electrical signal is comparatively small and hardly matters.
However, the conductor loss of the conductor increases with the
increase in frequency of the high-frequency electrical signal.
Therefore, the upper limit of the frequency at which the
propagation loss of a wire waveguide is no longer negligible is
estimated at several tens of terahertz.
[0017] In the present invention, the above-described two
electromagnetic wave transmission lines inside and outside the
conductor are terminated at the position of the aperture at the tip
of the conductor and are coupled with each other through the
aperture. Therefore, for example, when an analyte is located at a
distance of about the size of the minute aperture from the tip, the
change in the dielectric properties of the analyte gives a change
in the efficiency of the coupling. Therefore, typically, the
dielectric information of the analyte can be read by measuring the
high-frequency electrical signal passing through the minute
aperture. In other words, with the change in the dielectric
properties of the analyte, the impedance at the common terminal end
of the two electromagnetic wave transmission lines inside and
outside the conductor changes. Therefore, the amplitude and phase
of the reflected wave from one electromagnetic wave transmission
line to the other electromagnetic wave transmission line change. By
analyzing these, the dielectric properties of the analyte can be
evaluated. Since such an evaluation is useful typically for
obtaining a spatial resolution smaller than or equal to the
electromagnetic wave, the size of the minute aperture is smaller
than or equal to the wavelength. For example, in the case of a
high-frequency electrical signal in the terahertz band, the size of
the minute aperture is smaller than or equal to 1 mm. The size of
the minute aperture may be changed according to the spatial
resolution of the desired dielectric information of the
analyte.
[0018] The electromagnetic wave used in the present invention is
transmitted by a generator that is an appropriate electromagnetic
wave transmitting unit, and is received by a detector that is an
appropriate electromagnetic wave receiving unit. As described
above, in order to provide the reflection from one electromagnetic
wave to the other electromagnetic wave, it is most simple to
dispose the generator outside the tubular conductor and to dispose
the detector inside the tubular conductor. Alternatively, the
generator may be disposed inside the tubular conductor and the
detector may be disposed outside the tubular conductor. By just
determining the positional relationship between the generator and
the detector as above, the need for a directional coupler is
eliminated. According to the present invention, since a tubular
conductor is used as a waveguide and a wire waveguide, a probe and
a near-field microscope that require no directional coupler can be
provided, and the apparatus can be made comparatively simple. In
addition, since the propagation loss in the millimeter waveband to
the terahertz band can be reduced, the sensitivity of the probe and
the near-field microscope can be made comparatively high.
[0019] Further features of the present invention will become
apparent from the following description of exemplary embodiments
(with reference to the attached drawings).
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a sectional view showing the schematic
configuration of an embodiment of the present invention.
[0021] FIG. 2 is a sectional view showing the configuration of a
probe and a near-field microscope according to a first example of
the present invention.
[0022] FIG. 3 is a sectional view showing the configuration of a
probe and a near-field microscope according to a second example of
the present invention.
[0023] FIG. 4 is a sectional view showing the configuration of a
probe and a near-field microscope according to a third example of
the present invention.
[0024] FIG. 5 is a sectional view showing the observation of a
minute analyte with the probe and the near-field microscope
according to the first example.
[0025] FIG. 6 is a sectional view showing a modification of the
configuration of the probe and the near-field microscope according
to the first example.
DESCRIPTION OF THE EMBODIMENTS
[0026] An embodiment of the present invention will now be described
with reference to the drawings. The embodiment of a probe and a
near-field microscope includes a high-frequency transmission line
that is a tubular conductor having a minute aperture at the tip
thereof, and a generator and a detector respectively generating and
detecting a high-frequency electrical signal including part of the
frequency region from 30 GHz to 30 THz for measuring
characteristics of an analyte.
[0027] FIG. 1 is a sectional view showing the configuration of a
probe and a near-field microscope in this embodiment. In FIG. 1,
reference numeral 101 denotes a tubular conductor, and reference
numeral 102 denotes a minute aperture formed at the terminal end of
the tubular conductor 101. The size of the minute aperture 102 is
smaller than or equal to the wavelength of the electromagnetic wave
of the high-frequency electrical signal. The tubular conductor 101
is, for example, a circular waveguide or a rectangular waveguide.
The inner diameter of the tubular conductor 101 is, for example,
about the wavelength of the electromagnetic wave. The wall
thickness of the tubular conductor 101 is extremely thinner than
the inner diameter thereof, for example, about the skin depth of
the electromagnetic wave. An analyte 105 to be measured is disposed
near the minute aperture 102.
[0028] Reference numeral 103 denotes a high-frequency electrical
signal generator. In this embodiment, it is disposed outside the
tubular conductor 101. Reference numeral 104 denotes a
high-frequency electrical signal detector. In this embodiment, it
is disposed inside the tubular conductor 101. Reference numeral 106
shown by arrows outside and inside the tubular conductor 101
denotes a propagation path of the high-frequency electrical signal
due to the foregoing arrangement.
[0029] The propagation path 106 of the high-frequency electrical
signal will be described. Part of the high-frequency electrical
signal transmitted from the generator 103 is caught by the tubular
conductor 101. At this time, as shown in FIG. 1, the high-frequency
electrical signal propagates through the outer part of the tubular
conductor 101 (functioning as a wire waveguide) and reaches near
the minute aperture 102. Part of the high-frequency electrical
signal reflected by the analyte 105 passes through the minute
aperture 102, propagates through the inner part of the tubular
conductor 101 (functioning as a waveguide) as shown in FIG. 1,
reaches the detector 104, and is detected therein. In this way, the
reflection of the high-frequency electrical signal from the analyte
105 is measured.
[0030] In FIG. 1, the tubular conductor 101 is provided with a
taper structure 107 at the tip thereof in order to make the minute
aperture 102 smaller so as to obtain a higher resolution, and in
order to improve the efficiency of the propagation in the two
high-frequency transmission lines inside and outside the conductor
101, which are coupled via the minute aperture 102. The taper
structure 107 may be a multistage taper structure. The minute
aperture 102 may be circular or rectangular. The size of the
aperture, which provides the spatial solution, is preferably nearly
equal to the spatial frequency of the dielectric properties of the
analyte or the size of the minute region of the analyte. In order
to efficiently couple the high-frequency electrical signal
transmitted from the generator 103 to the tubular conductor 101,
the high-frequency electrical signal may be appropriately collected
with an optical device such as a lens, and the tubular conductor
101 may be irradiated therewith.
[0031] In this embodiment, the high-frequency electrical signal
obtained in the detector 104 can be separated as follows. That is
to say, the high-frequency electrical signal passing through the
minute aperture 102 can be separated into the high-frequency
electrical signal that is not related to the presence or absence of
the analyte 105 and the high-frequency electrical signal that is so
related. The proportion of these is the contrast obtained in the
detector 104. The high-frequency electrical signal that is related
to the presence or absence of the analyte 105, that is to say, the
reflection from the analyte depends on the distance between the
minute aperture 102 and the analyte 105, the complex dielectric
constant of the analyte 105, and the shape of the analyte 105.
Therefore, for example, if scanning is performed with the distance
between the minute aperture 102 and the analyte 105 maintained
constant, an image according to the complex dielectric constant
distribution of the skin structure of the analyte 105 can be
obtained. In this case, when the distance between the minute
aperture 102 and the analyte 105 is nearly equal to the size of the
minute aperture 102, an excellent contrast is obtained. The
information thus obtained in the detector 104, such as the phase
retardation and the amplitude of the high-frequency electrical
signal, is sent to a PC (not shown) and analyzed.
[0032] The high-frequency electrical signal transmitter 103 is
selected according to, for example, the frequency region of the
complex dielectric constant of the analyte to be observed. If the
frequency region is the millimeter waveband or the submillimeter
waveband, it may be a Gunn oscillator using a Gunn diode. In this
case, the high-frequency electrical signal detector 103 may be a
Schottky barrier diode for detection. If the frequency region is
the terahertz band, photoconductive antennas can be used as the
transmitter and the detector. The transmitter may also be a BWO
(Backward Wave Oscillator), a quantum cascade laser, or a resonant
tunneling diode, and the detector may also be a pyroelectric
element or a Golay cell.
[0033] In accordance with the purpose of the invention, the
outer-wall structure of the tubular conductor can have a dielectric
coating for reducing the propagation loss of the high-frequency
electrical signal. Although the inside of the tubular conductor is
preferably hollow (that is to say, air) from the viewpoint of
efficient electromagnetic wave propagation, the inner structure of
the above-described tubular conductor may be filled with a
dielectric material having a small dielectric tangent. The
dielectric tangent (tan .delta.) of the dielectric material is
preferably 0.1 or less. In this case, manufacturing is easy, and
the size can be reduced to be several times smaller. In addition, a
high-frequency electrical signal coupling unit, such as a cross
wire, may be disposed in the outer structure of the tubular
conductor. This is used when the high-frequency electrical signal
generator or the high-frequency electrical signal detector is
located in a free space, in order to reduce the frequency
dependency of the efficiency of coupling the outer structure (wire
waveguide) of the tubular conductor and the free space, or in order
to improve the coupling efficiency itself for a particular
frequency.
EXAMPLES
[0034] Examples of specific configurations are as follows.
First Example
[0035] FIG. 2 is a sectional view showing a first example of a
probe and a near-field microscope according to the present
invention. In FIG. 2, reference numeral 101 denotes a circular
waveguide filled with a dielectric material 111, and reference
numeral 102 denotes a minute aperture at the terminal end of the
circular waveguide 101. Also in this case, as described above, an
analyte 105 is disposed near the minute aperture 102.
[0036] Reference numeral 103 denotes a terahertz wave generator,
which in this example is a photoconductive antenna. Reference
numeral 104 denotes a terahertz wave detector, which is also a
photoconductive antenna. Reference numeral 107 denotes a taper
structure formed at the tip of the circular waveguide 101.
Reference numeral 106 denotes the flow of the terahertz wave due to
the arrangement of the above elements.
[0037] In this example, the inner diameter of the circular
waveguide 101 is 1 mm, and the dielectric material 111 is Teflon
(registered trade name), whose dielectric constant is about 2, and
whose dielectric tangent (tan .delta.) is comparatively small.
Therefore, the cutoff frequency of the circular waveguide 101 in
the TE11 mode is calculated at about 0.12 THz, and most of the
frequency region (0.1 THz or more) of the terahertz wave generated
from a typical photoconductive antenna can be used.
[0038] As in this example, in order to obtain the information of
both the phase retardation and the amplitude of the terahertz wave
in the detector 104, an optical delay device 205 and a beam
splitter 202 may be provided. That is to say, a configuration
including a delay device or a heterodyne detector for measuring the
phase difference between the reflected wave from the analyte and
the original high-frequency electrical signal can be selected.
[0039] In this case, a femtosecond laser light generator 201
injects light into the terahertz wave generator 103 and the
terahertz wave detector 104 via optical fibers 203 and 204,
respectively. Since the terahertz wave radiated from the terahertz
wave generator 103 is comparatively broadband, if a cross-wire
terahertz wave coupling unit 112 disclosed in Nature, vol. 432
(2004) is used, the coupling efficiency will be flat in a
comparatively wide frequency region. This coupling unit 112 is, for
example, a metal wire extending in the direction perpendicular to
the plane of FIG. 2.
[0040] This example operates, for example, as follows. The distance
between the minute aperture 102 side end and the detector 104 side
end of the circular waveguide 101 is, for example, 3 mm or more.
The resonance frequency of the circular waveguide 101 in the
vertical direction in FIG. 2 is smaller than the product of the
frequency of the terahertz wave to be used and 1/Q (Q is the
resonator Q value) of the circular waveguide resonator. In this
case, the reflection of the terahertz wave from the analyte 105
negligibly produces a standing wave. In this way, information such
as the phase retardation (the imaginary part of the dielectric
constant) and the amplitude (the real part of the dielectric
constant) of the reflected wave with the dielectric properties of
the analyte 105 can be measured as the dielectric properties of the
analyte 105 in a comparatively wide frequency region. That is to
say, when information of the analyte is obtained using the above
probe or near-field microscope, the information of the analyte can
be obtained by making an electromagnetic wave that is a traveling
wave exist in the inner part of the tubular conductor.
[0041] Alternatively, if the distance between the minute aperture
102 side end and the detector 104 side end of the circular
waveguide 101 is, for example, about 3 mm, the resonance frequency
of the circular waveguide 101 in the vertical direction in FIG. 2
appears about every 35 GHz. Therefore, peaks are detected at every
integral multiple of this value. The dielectric properties of the
analyte changes the resonator Q value or shifts the resonance
frequency. Therefore, this method may be used to measure the
dielectric properties of the analyte 105 at a particular frequency.
Thus, when information of the analyte is obtained using the above
probe or near-field microscope, the information of the analyte can
also be obtained by making an electromagnetic wave that is a
standing wave exist in the inner part of the tubular conductor.
[0042] In this example, the circular waveguide 101 filled with the
dielectric material 111 is manufactured so that the tip of a Teflon
tube of 1.0 mm in diameter is dissolved by chemical etching so as
to be sharpened. Next, the Teflon tube is coated by vapor
deposition with, for example, gold. The coating may be silver,
copper, aluminum, brass, or nickel. The skin depth for gold of the
terahertz wave used in this example is several tens of nanometers
to several hundred nanometers. Therefore, coupling of the two
high-frequency transmission lines inside and outside the circular
waveguide 101 can be prevented with a submicron film thickness (for
example, 300 nm). In addition, in order to make the minute aperture
102, the tip is cut, and chemical etching is performed. To the
other end of the thus made circular waveguide 101 filled with the
dielectric material 111, a photoconductive antenna functioning as
the terahertz wave detector 104 is attached, for example, with an
epoxy adhesive. The probe according to this example can be
manufactured, for example, through the above well-known
process.
[0043] FIG. 5 shows another example to observe an analyte in the
first example. In FIG. 5, reference numeral 501 denotes a minute
analyte, for example, a DNA, which is in the submicron scale.
Reference numeral 502 denotes a holder for the minute analyte 501,
for example, a semiconductor wafer. Since the characteristic of
molecular vibration in a DNA appears in the frequency region of the
terahertz band, this example can be used, for example, to
distinguish a plurality of DNAs with different structures by their
characteristics of molecular vibration. Since the desired spatial
resolution is nearly equal to the scale of the DNAs, the diameter
of the minute aperture 102 suitable for observing each DNA is 300
nm.
[0044] In the first example, the photoconductive antenna serving as
the terahertz wave generator 103 is, as is well known, supplied
with a bias voltage to generate a terahertz wave. In the
photoconductive antenna serving as the terahertz wave detector 104,
as is also well known, a current flowing into the photoconductive
antenna is detected. If the operation of the terahertz wave
generator 103 in FIG. 2 is exchanged for the operation of the
terahertz wave detector 104 in FIG. 2, the high-frequency
electrical signal 106 flows in the reverse direction. That is to
say, the terahertz wave generator 103 in FIG. 2 functions as a
terahertz wave detector, and the terahertz wave detector 104 in
FIG. 2 functions as a terahertz wave generator, as shown in FIG. 6.
The dielectric properties of the analyte can also be measured in
this way. In FIG. 6, reference numeral 112 denotes the foregoing
cross-wire terahertz wave coupling unit. The high-frequency
electrical signal 106 propagating in the outer part of the circular
waveguide 101 is directed to the detector 104 by the cross-wire
terahertz wave coupling unit 112.
[0045] The above-described first example can simplify the apparatus
and can provide a probe and a near-field microscope that have a
comparatively high sensitivity in the frequency region from the
millimeter waveband to the terahertz band.
Second Example
[0046] FIG. 3 is a sectional view showing a second example of a
probe and a near-field microscope according to the present
invention. In FIG. 3, reference numeral 101 is a rectangular
waveguide filled with a dielectric material 111, and reference
numeral 102 denotes a minute aperture formed at the terminal end of
the rectangular waveguide 101. Reference numeral 103 denotes a
terahertz wave generator. In this example, the terahertz wave
generator 103 is a continuous wave light source (a BWO, a quantum
cascade laser, or a resonant tunneling diode). Reference numeral
104 denotes a terahertz wave detector. In this example, the
terahertz wave detector 104 is a pyroelectric element. Reference
numeral 106 denotes the flow of the terahertz wave due to this
arrangement. In addition, in order to image an analyte 105 disposed
near the minute aperture 102, this example includes a
probe-position control system 301 configured to make the probe
according to this example scan the analyte 105.
[0047] The probe-position control system 301 is configured using
well-known methods. For example, as shown in FIG. 3, the system 301
includes a laser light generator 302, a photodiode array 303, a
position detector 304, and an actuator 305 and feedback-controls
the relative positional relationship between the minute aperture
102 and the analyte 105. Part of the light from the laser light
generator 302 reflected by the side surface of the rectangular
waveguide 101 is detected by the photodiode array 303. On the basis
of the amount of displacement of the detected light, position
information can be obtained. At this time, the relative position
information between the minute aperture 102 and the analyte 105 is
input into the position detector 304, and the actuator 305 corrects
the displacement from a predetermined position.
[0048] The position detection using laser light enables a control
on a submicron scale. In addition to the above probe position
control system, in order to accurately maintain a constant distance
between the minute aperture 102 and the analyte 105, a position
control method using atomic force, tunneling current, or floating
capacitance may be used. Thus, imaging can be achieved by obtaining
and processing both the relative position between the minute
aperture 102 and the analyte 105, and by the reflection of the
terahertz wave from the analyte 105.
[0049] In this example, since a continuous wave light source is
used as the terahertz wave generator 103, monochromaticity is
comparatively high. In the case where the monochromaticity of the
terahertz wave radiated from the terahertz wave generator 103 is
high, the coupling efficiency can be further improved with a metal
grating terahertz wave coupling unit 112 shown in FIG. 3 so as to
further improve the S/N ratio (signal-to-noise ratio) of the
detection signal.
[0050] This imaging is used, for example, for observing the carrier
concentration distribution in a semiconductor wafer. The plasma
frequency estimated from a typical carrier concentration of
semiconductor wafers is located in the frequency region of the
terahertz band. The complex dielectric constant changes drastically
near the plasma frequency. Therefore, the probe and the near-field
microscope of this example is used, for example, for observing the
carrier concentration distribution in a semiconductor wafer with
higher spatial resolution.
[0051] In this example, the rectangular waveguide 101 can be
manufactured using the same method as described in the first
example. The pyroelectric element 104 is comparatively small-sized
and is, as shown in FIG. 3, fitted into the other terminal end of
the rectangular waveguide 101. The metal grating terahertz wave
coupling unit 112 can be manufactured through a well-known process
including, for example, application of photoresist, development,
and plasma etching.
Third Example
[0052] FIG. 4 is a sectional view showing a third example of a
probe and a near-field microscope according to the present
invention. The third example shown in FIG. 4 is an example in which
a tubular conductor 101 is inserted into an analyte 401 having an
inner structure of the above-described embodiment of the present
invention. Therefore, this example can measure not only the
dielectric properties in the skin structure of the analyte 401 but
also the dielectric properties of the inner structure of the
analyte 401. The analyte 401 is preferably an object that does not
make the coupling between the outer part and the inner part of the
tubular conductor 101 through the minute aperture 102 too small.
Therefore, the analyte 401 is preferably an object that does not
have conducting properties. In addition, in order to prevent the
decrease in sensitivity, the analyte 401 is preferably a dielectric
material whose dielectric tangent (tan .delta.) is small (for
example, 0.1 or less).
[0053] This example is used, for example, for observing the
three-dimensional distribution of the dielectric properties in a
rubber material. It is well known that when sulfur is added to a
rubber material in order to cause cross-linking, the sulfur binds
chemically to other molecules and effects a change in complex
dielectric constant. The characteristic also appears in the
frequency region of the terahertz band. The generator 103
preferably generates a high-frequency electrical signal at a
frequency (or in a frequency region) at (or in) which the complex
dielectric constant due to the chemical bond between sulfur and
other molecules changes drastically. In this case, the contrast is
improved. For example, if the probe is appropriately scanned or
swept using the probe position control system described in the
second example, the three-dimensional spatial distribution of the
dielectric properties of the rubber material can be observed.
[0054] While the present invention has been described with
reference to exemplary embodiments, it is to be understood that the
invention is not limited to the disclosed exemplary embodiments.
The scope of the following claims is to be accorded the broadest
interpretation so as to encompass all modifications, equivalent
structures and functions.
[0055] This application claims the benefit of Japanese Application
No. 2006-073614 filed Mar. 17, 2006, which is hereby incorporated
by reference herein in its entirety.
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